MRI apparatuses are diagnostic imaging apparatuses for medical use, which induce magnetic resonance of atomic nuclei in an arbitrary section crossing a subject, and obtain a tomographic image of the section from generated magnetic resonance signals. They transmit a radio frequency wave (henceforth abbreviated as RF), which is a kind of electromagnetic wave, to a subject to excite spins of atomic nuclei in the subject, then receive magnetic resonance signals generated by nuclear spins, and obtain an image of the subject. RF is transmitted to the subject by a coil for RF transmission, and the magnetic resonance signals emitted by the subject are received by a coil for RF reception.
In recent years, aiming at improvement in SNR (Signal to Noise Ratio) of images, the static magnetic field intensity tends to become higher, and use of a high magnetic field MRI apparatus (super-high magnetic field MRI apparatus) using a static magnetic field intensity of 3 T (tesla) or higher has begun to spread. However, although higher static magnetic field intensity provides more improved SNR, higher static magnetic field intensity more easily produces non-uniformity in obtained images. This is because the frequency of RF used for inducing the magnetic resonance phenomenon becomes higher with use of higher magnetic field intensity. For example, for an MRI apparatus using a static magnetic field intensity of 3 T (tesla) (henceforth referred to as 3 T MRI apparatus), RF having a frequency of 128 MHz is used. In human living bodies, the wavelength of this RF is about 30 cm, which is substantially the same scale as that of a section of the abdominal part, and the phase thereof changes. This phase change makes the distribution of the irradiated RF and the spatial distribution of the rotating magnetic field for inducing the magnetic resonance phenomenon, which is generated by RF (henceforth referred to as radio frequency magnetic field distribution, B1), non-uniform, and produces non-uniformity in images, as a result. Therefore, there is required a technique for reducing the non-uniformity of the distribution of the rotating magnetic field B1 at the time of the RF irradiation performed by a super high magnetic field MRI apparatus.
As an RF irradiation method for reducing the non-uniformity of the B1 distribution, there is a technique called “RF shimming”. This is a technique for reducing the B1 non-uniformity of imaging region by using a coil for transmission having multiple channels, and controlling phases and amplitudes of RF pulses applied to the channels (for example, refer to Patent document 1). In this technique, the B1 distribution of each channel is measured beforehand before the main scan, and by using this B1 distribution, amplitude and phase of RF pulse optimal for reducing the B1 non-uniformity are calculated. In this operation, a region desired to be diagnosed is set as a region of interest (ROI), and amplitude and phase are determined so as to reduce the B1 non-uniformity in the ROI.
There has also been proposed a method for irradiating RF, in which the B1 value is made high only in ROI, and the B1 value is made low in the other regions (for example, refer to Non-patent document 1). In this method, amplitude and phase of RF are set so that the B1 value is maximized in ROI by using a ratio of B1 averages of ROI and the regions other than ROI. The B1 distribution is thereby locally concentrated in ROI.
Furthermore, there has also been proposed a method of highly precisely controlling the B1 distribution by changing the RF waveform and gradient magnetic field waveform (for example, refer to Patent document 2).